Semiconductor deep-ultraviolet (DUV) light-emitting diodes (LEDs) operating at sub-250 nm wavelengths have attracted interest due to their applications in such areas as sterilization, biosensing, medical treatment, and lithography. Aluminum gallium nitride (AlGaN) is a widely used material for commercial UV LEDs because of its direct bandgap spanning the UVA, UVB, and UVC regions of the electromagnetic spectrum. However, at sub-250 nm wavelengths, challenges arise, among which the key issues include. 1) degrading crystal quality of the high-Al-content AlGaN materials, as defects related to non-radiative recombination can severely degrade internal quantum efficiency (IQE); 2) low conductivity and poor carrier injection of the AlGaN, especially for p-type AlGaN, due to the large ionization energy of donors (280 meV for AlN:Si), and the even larger ionization energy of acceptors (630 meV for AlN:Mg); and 3) compromised light extraction, especially at DUV wavelengths, due to absorption by the p-GaN contact layer, which is commonly adopted to form a highly conductive ohmic contact.
Planar DUV LEDs are commonly grown on highly lattice-mismatched substrates, resulting in a high concentration of threading dislocations (TDs), which act as non-radiative recombination centers that contribute to low internal quantum efficiencies. Monolayer gallium nitride (GaN) quantum wells and dots between AlN barriers have been demonstrated to improve IQE, as carriers are kept away from non-radioactive recombination centers due to three-dimensional confinement. (See, Kandaswamy. et al. Tunnel-injection GaN quantum dot ultraviolet light-emitting diodes. Appl Phys Lett 102, 041103 (2013); Taniyasu Y., et al., Polarization property of deep-ultraviolet light emission from C-plane AlN/GaN short-period superlattices. Appl Phys Lett 99, 251112 (2011); Verma et al. Tunnel-injection quantum dot deep-ultraviolet light-emitting diodes with polarization-induced doping in III-nitride heterostructures. Appl Phys Lett 104, 021105 (2014); and Islam et al. MBE-grown 232-270 nm deep-UV LEDs using monolayer thin binary GaN/AlN quantum heterostructures. Appl Phys Lett 110, 041108 (2017).) Light emission at 232 nm by 1-2 monolayer (ML) GaN quantum structures has been reported. This is probably the shortest wavelength this GaN/AlN quantum heterostructure could achieve, considering the theoretical limit and single monolayer GaN growth precision. In addition, the light emission efficiency suffers from the absorption and re-emission of n- and p-AlGaN (Al: 0.5 to 1) regions with lower bandgap energy (4.6 for Al0.5Ga0.5N) than the 232 nm photon.
To improve free carrier concentration in high-Al-content AlGaN materials, researchers have developed polarization doping, the purpose of which is to enhance electrical conductivity and hole injection. (See. Islam et al. MBE-grown 232-270 nm deep-UV LEDs using monolayer thin binary GaN/AlN quantum heterostructures. Appl Phys Lett 110, 041108 (2017); Simon, et al. Polarization-induced hole doping in wide-band-gap uniaxial semiconductor heterostructures. Science 327, 60-64 (2010); and Ambacher, et al. Two-dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures. J Appl Phys 87, 334-344 (2000).) Doping by varying the Al composition and the corresponding polarization strength has been employed to take advantage of the intrinsic spontaneous polarization effect to increase carrier concentration. (See. Jena. et al. Realization of wide electron slabs by polarization bulk doping in graded III-V nitride semiconductor alloys. Appl Phys Lett 81, 4395-4397 (2002); Li. et al. Polarization induced hole doping in graded AlxGa1-xN (x=0.7˜1) layer grown by molecular beam epitaxy. Appl Phys Lett 102, 062108 (2013); Neufeld. et al. Effect of doping and polarization on carrier collection in InGaN quantum well solar cells. Appl Phys Lett 98, 243507 (2011); Sanchez-Rojas, et al. Tailoring of internal fields in AlGaN/GaN and InGaN/GaN heterostructure devices. Phys Rev B 61, 2773-2778 (2000); and Kozodoy, et al. Polarization-enhanced Mg doping of AlGaN/GaN superlattices. Appl Phys Lett 75, 2444-2446 (1999).) For the [000-1] crystallographic orientation (N-face), the Al mole fraction, x, in the AlxGa1-xN needs to be graded up towards the p and n contacts, in which case the x grading range required for effective doping is limited. Ultimately, this method is not applicable when the Al composition in the active quantum barriers approaches 1, which is necessary in order to generate high energy photons (e.g., ˜5 eV for 250 nm). The Ga-face ([0001]) requires the Al composition to grade down towards the contacts, which, if graded down to the point that the band gap energy is smaller than that of the emitted photon, will act as a highly absorptive layer due to smaller band gap energy.